Light-emitting diode (LED) is known by its high energy efficiency and so introduced to the energy-saving movement in many kinds of equipments of variable fields including outdoor lighting, means of transportation, and household lighting, such as street light, traffic light, outdoor display, headlamp, trail light, center high mounted stop lamps (CHMSL), and decoration light.
A basic structure of a light-emitting diode chip mainly includes a p-n junction. The hole of the p-type semiconductor material and the electron of the n-type semiconductor recombine to emit light under a bias voltage. The light-emitting area in the structure is sometimes called “active layer” or “light-emitting layer”. The wavelength from the light-emitting layer is determined by the adopted material. The structure emitting red light is constructed by introducing a main material such as GaP, GaAsP, AlGaAs, and AlGaInP. The structure emitting green light is constructed by introducing a main material such GaP and ZnCdSe. The structure emitting blue light is constructed by introducing a main material such as SiC and GaN. Those different materials are usually grown on different substrates such as GaP, GaAs, SiC, and sapphire.
The available light-emitting diode chip in market emits not only white light but also variable color lights of almost the whole range of the visible wavelength (400 nm˜750 nm) and ultraviolet. White light can be generated in several ways such as by mixing lights from blue, green, and red chips, exciting phosphor by UV light, exciting phosphor by blue light, using semiconductor wavelength converting material (also called “photo-recycling semiconductor LED”; PRS LED), and dye. The most common commercial way of generating white light makes yttrium aluminum garnet (YAG) phosphor be pumped by blue light to generate a complementary color. For example, a 460 nm blue chip is introduced to excite YAG:Ce phosphor to generate a 570 nm around yellow light, one may control the concentration and the thickness of the phosphor to adjust the ratio of blue light to yellow light in order to produce white lights having various color temperatures.
Phosphor absorbs shorter wavelengths to emit longer wavelengths, that is, absorbs a high energy level radiation to emit a low energy level radiation. A phosphor is characterized by its excitation band and emission band. The excitation band has a primary wavelength distribution shorter than that of the emission band, while the two bands may overlap in part. The peak wavelength difference of the absorption band and the emission wavelength is called “Stokes shift”. The phosphor may be caused to radiate in a similar emission spectrum by any wavelength within the excitation band. However, the phosphor is operated at different efficiencies responsive to different absorbed wavelengths, which depends on the composition of phosphor.
In another aspect, the numerous chips on a wafer appear to be in a non-uniform wavelength distribution spanning from 10 nm to 20 nm or more. Provided a specific recipe of phosphor is introduced to directly or indirectly overlay on or be packaged with all chips, the color temperature of the white light from the end product is divergent so significantly that the qualitative uniformity of the application product is affected.
As shown in FIG. 1A, LED light source 12 emits blue lights 11 and 13 at specific wavelengths. The blue light 13 excites phosphor 14 to generate yellow light 15. The blue light 11 and the yellow light 15 are mixed into white light 17. The yellow light 15 is going to remain in the same spectrum even if the wavelength of the blue light from the LED light source 12 is changed but still within the excitation band of the phosphor 14. The wavelength difference of the blue light therefore results in a shift of the color temperature of the mixed white light 17.
In addition, a CIE chromaticity diagram is shown in FIG. 1B. A 460 nm blue light and a 571 nm yellow light generated by exciting YAG phosphor at a fixed condition are mixed into a white light of about 6000K color temperature. However, if the blue light wavelength shifts up to 470 nm or down to 450 nm, while the yellow light is unchanged, the color temperature of the mixed white light also shifts to 10000K or 5000K, which is usually unacceptable to common applications. To produce color lights having constant color temperatures, therefore, the chips are necessarily undergone sorting and binning processes before a condition- or characteristic-specific phosphor is applied to.